Constructed wetlands with horizontal subsurface flow (HF CWs) are primarily designed to remove organics and suspended solids. The major principle of this treatment process is that wastewater flows through the porous medium planted with wetland vegetation in order to achieve removal of pollutants. During the process the void spaces in the filtration material are slowly filled due to sedimentation and filtration of suspended solids, creation of biofilms on filtration material particles, chemical precipitation and growth of roots and rhizomes of the vegetation. Therefore, the clogging process is inevitable. However, the use of suitable porous filtration materials, responsible maintenance of pretreatment units and suitable loadings of organics and suspended solids may greatly slow down the clogging process. The purpose of this study was to evaluate the effect of partial ponding on treatment performance of four HF CWs that have been in operation for at least 20 years. The results revealed that if the inflow loading rates are < 10 g BOD5 m(-2) d(-1), < 20 g COD m(-2) d(-1), < 10 g TSS m(-2) d(-1), the partial ponding may occur after about 15 years of operation. The results also revealed that partial ponding have no significant effect on the quality of discharged water and the replacement of inflow zone filtration material did not results in any significant improvement either.